Adaptive echo cancellation is provided in telephony and data communication engineering to prevent echo signals from affecting the reception. For example, in a data modem connected to a two-wire line, the transmitter in one of the two-wire directions and the receiver in the other are connected to the two-wire line via a hybrid coupler, data (b.sub.k) being sent from the transmitter across the line to a modem at the remote end, while conversely, data (a.sub.k) is transmitted from the remote end across the line to the local modem receiver. Due to deficiencies in the hybrid coupler, it is unavoidable that a certain proportion of data flow (b.sub.k) from the transmitter passes through the hybrid coupler and into the receive path, reception of the data flow (a.sub.k) thus being disturbed. Furthermore, disturbing signals occur in the form of echoes from the local transmission at the remote end, but the leak signals through the coupler are the ones which dominate and which most heavily affect the reception. These leak signals coming from the local data flow (b.sub.k) and occurring in the local modem detector as disturbance signals are usually called echo signals, even if they have not been transmitted across the line, reflected and transmitted back again to the local modem.
In order to reduce the effect of such echo signals it is known in the prior art to provide a digital-type balance filter, usually a finite impulse response (FIR) filter connected to the transmit and receive channels. The task of the balancing filter is accordingly to form a signal from the transmitting data flow, this signal being subtracted from the one which occurs at the detector input, after having passed through the hybrid coupler, and which contains leak signals from the transmit channel. For rapidly adjusting the balance filter parameters, i.e. provide rapid convergence in the balance filter, it is however required that the correlation between the incoming analogue signal in the receive path (denoted below w(t)+h(t)) and the transmitted data flow b.sub.k be great. The presence of the remote signal w(t) coming from the data flow (a.sub.k) decreases this correlation however, and convergence is slow.
Rapid convergence of the balance filter thus requires that the analogue remote signal w(t) be eliminated in some way when calculating the parameter adjustment. A known method of eliminating or cancelling signal w(t) is to ensure that the remote end transmitter is disconnected during a test period when the adjustment of the balance filter can be carried out. Transmission quality will then be entirely dependent on how successful the adaption has been during the test period. The test period must be made relatively long. It would be more effective if signal w(t) could be subtracted from incoming signal w(t)+h(t). In digital transmission signal w(t) only attains a limited number of amplitude values at the sampling instants (if intersymbol interference is neglected). An estimation w(k) of signal w(t) can therefore be made in the sampling instants k with the aid of a quantizer.
Since the cable attenuation is normally not known, some form of automatic level adjustment must be made for obtaining a good estimate. Different systems with adaptive balance filters and adaptive level adjustments have been described, e.g. in "Adaptive Echo Cancellations/AGC Structures for Two Wire, Full Duplex Data Transmission", Bell System Techn. Journal 58 No. 7 (September, 1979), the level adjustments utilize correlation of the received signal with a level estimation a.sub.k of the data sequence transmitted from the remote end. The methods operate if this estimation is more or less correct. The leak signal h(t) is however much larger than signal w(t) in many practical cases. When the balance signal r(t) passes the heavily non-linear quantizer incorporated in the receiver, all the information concerning signal w(t) may be lost if the balance filter is not correctly adjusted. In such cases a.sub.k will be more heavily correlated to data flow b.sub.k than to data flow a.sub.k. This has the practical result that the adaption of the balance filter and adaption of the level counteract each other and convergence cannot be obtained.